Would it be permissible to use the Water Wow! toy on Shabbat? Assuming that it is, at the very least, uvda d’chol, what are other issues are there?
Here’s a description of how it works:
The basic concept is that each page is “laminated” with a very thin sheet of paper – almost like tissue paper, but more durable. When the paper is moistened by the water in the pen, it becomes see-through and reveals the picture underneath. There’s no chemical reaction involved.